The present invention relates to a focus-adjustment signal generating apparatus and method for generating a video signal which is displayed on a view finder (VF), a liquid crystal display (LCD), etc., for manual focus adjustments, an imaging apparatus, such as a video camera and an electronic still camera, equipped with such a focus-adjustment signal generating apparatus, and an imaging method combined with such a focus-adjustment signal generating method.
Recent video cameras and electronic still cameras offer higher-resolution images. Nevertheless, VF and LCD offer lower image resolution to users when users check the images to be taken through the VF or LCD. This is because of limited VF or LCD sizes in compact video cameras or electronic still cameras. Such lower image resolution on VF or LCD renders focus adjustments difficult for users while viewing images through VF or LCD.
A known focus-adjustment signal generating apparatus is disclosed, for example, in Japanese Un-examined Patent Publication No. 2006-58683, for easier focus adjustments with clear images through VF or LCD. The known apparatus displays an image of an object on VF or LCD with edge processing. The edge processing is performed as follows: A luminance signal is extracted from an input video signal of the object. An R (red) signal, a G (green) signal and a B (blue) signal, or the three primary color signals (RGB signals), are generated based on the luminance signal. The luminance signal is subjected to high-pass filtering to output high-frequency components in the horizontal, vertical and diagonal directions. The high-frequency components in the horizontal, vertical and diagonal directions are added to the R signal, the G signal and the B signal, respectively, to put a specific color on the image of the object at the high-frequency components.
Accordingly, the known focus-adjustment signal generating apparatus offers an image of an object on VF or LCD with a specific color on edge portions of the image. Focus adjustment is performed to make the specific color darker for sharper edge portions. This offers easier focus adjustments with clear images through VF or LCD.
In such known focus-adjustment signal generating apparatus, the difference between the resolution of a video signal gained through image taking and that of VF or LCD forces a focus-adjustment signal to lose high-frequency components when generated after down-conversion, resulting in an incorrect focus-adjustment signal. It is thus required to generate the focus-adjustment signal before down-conversion when the resolution is different between the video signal and VF or LCD. The difference in resolution occurs, for example, when a high-resolution high-vision (HD) signal gained through image taking is down-converted to a standard-resolution standard (SD) signal on VF or LCD.
In the entire camera system, signals to be processed around a down converter are not an RGB signal but luminance and chrominance difference signals for most video cameras and electronic still cameras. The RGB signal to be used in generation of a focus-adjustment signal is processed in a later stage in the entire camera system, compared to the luminance and chrominance difference signals in VF or LCD in the known focus-adjustment signal generating apparatus described above. The generation of a focus-adjustment signal thus requires an RGB converter in a later stage in the entire camera system, causing the system processing redundant.
The high-frequency components of the luminance signal required for generation of a focus-adjustment signal correspond to the edge portions of an image. A lower area ratio of the edge portions to the image could force a down-converted focus-adjustment signal to lose such high-frequency components. Thus, no processing to the image edge portions very likely cause inaccurate focus adjustments.
Moreover, extraction of high-frequency components from a luminance signal at a fixed reference level cannot give a dynamic focus-adjustment signal due to change in the ratio of low- to high-frequency components depending on images.